U.S. patent number 7,297,395 [Application Number 10/461,052] was granted by the patent office on 2007-11-20 for superabsorbent materials having low, controlled gel-bed friction angles and composites made from the same.
This patent grant is currently assigned to Kimberly-Clark Worldwide, Inc.. Invention is credited to Richard Norris Dodge, II, Joseph Raymond Feldkamp, Arvinder Pal Singh Kainth, Stacy Averic Mundschau, Estelle Anne Ostgard.
United States Patent |
7,297,395 |
Kainth , et al. |
November 20, 2007 |
Superabsorbent materials having low, controlled gel-bed friction
angles and composites made from the same
Abstract
The present invention relates to water swellable, water
insoluble superabsorbent materials having controlled variable
gel-bed friction angles. Controlling the gel-bed friction angle of
the superabsorbent materials may allow control of the swelling of
the material, the absorbency of the material, and/or the
absorbency, resiliency, and porosity of the absorbent composite
containing the superabsorbent material. The present invention
relates to treatments for superabsorbent materials to manipulate
friction angle and new superabsorbent materials having the desired
friction angle characteristics. The present invention also relates
to absorbent composites employing superabsorbent materials having
the desired friction angle characteristics.
Inventors: |
Kainth; Arvinder Pal Singh
(Neenah, WI), Dodge, II; Richard Norris (Appleton, WI),
Feldkamp; Joseph Raymond (Appleton, WI), Mundschau; Stacy
Averic (Oshkosh, WI), Ostgard; Estelle Anne (Appleton,
WI) |
Assignee: |
Kimberly-Clark Worldwide, Inc.
(Neenah, WI)
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Family
ID: |
31191324 |
Appl.
No.: |
10/461,052 |
Filed: |
June 13, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040030312 A1 |
Feb 12, 2004 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60399877 |
Jul 30, 2002 |
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Current U.S.
Class: |
428/292.1;
442/100; 442/118; 442/414; 442/417; 442/97; 442/99 |
Current CPC
Class: |
A61F
13/53 (20130101); A61L 15/42 (20130101); A61L
15/60 (20130101); A61F 2013/530708 (20130101); Y10T
442/2328 (20150401); Y10T 442/699 (20150401); Y10T
428/249924 (20150401); Y10T 442/2484 (20150401); Y10T
442/696 (20150401); Y10T 442/2336 (20150401); Y10T
442/2311 (20150401) |
Current International
Class: |
B32B
7/08 (20060101); B32B 27/04 (20060101) |
Field of
Search: |
;428/281,283,292.1
;604/368 ;442/97,99,100,118,414,417 |
References Cited
[Referenced By]
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Primary Examiner: Dye; Rena
Assistant Examiner: Thompson; Camie S.
Attorney, Agent or Firm: Rosiejka; Bryan R.
Parent Case Text
This application claims priority from U.S. Provisional Application
No. 60/399,877 filed on Jul. 30, 2002.
Claims
We claim:
1. A superabsorbent material, comprising: a water swellable, water
insoluble superabsorbent material; and, the superabsorbent material
having a first gel-bed friction angle at a superabsorbent material
swelling level of about 2.0 grams of 0.9 weight percent sodium
chloride solution/gram of the superabsorbent material and gel-bed
friction angles, at superabsorbent material swelling levels greater
than about 2.0 grams of 0.9 weight percent sodium chloride
solution/gram of the superabsorbent material, substantially equal
to or less than the first gel-bed friction angle, wherein the first
gel-bed friction angle is about 20 degrees or less.
2. The superabsorbent material of claim 1, wherein the first
gel-bed friction angle is about 10 degrees or less.
3. The superabsorbent material of claim 1, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of natural materials, modified
natural materials, synthetic materials, and combinations
thereof.
4. The superabsorbent material of claim 3, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of silica gels, agar, pectin, guar
gum, alkali metal salts of polyacrylic acids, polyacrylamides,
polyvinyl alcohols, ethylene maleic anhydride copolymers, polyvinyl
ethers, hydroxypropylcelluloses, polyvinyl morpholinones, polymers
and copolymers of vinyl sulfonic acid, polyacrylates,
polyacrylamides, polyvinyl pyridine, acrylonitrile grafted starch,
acrylic acid grafted starch, isobutylene maleic anhydride
copolymers, polyamines, and combinations thereof.
5. The superabsorbent material of claim 1, further comprising a
friction angle reducing additive in combination with the
superabsorbent material.
6. The superabsorbent material of claim 5, wherein the friction
angle reducing additive is selected from the group consisting
essentially of glycerol, mineral oil, silicone oil,
polysaccharides, polyethylene oxides, and combinations thereof.
7. The superabsorbent material of claim 5, further comprising an
emulsifier in combination with the superabsorbent material.
8. The superabsorbent material of claim 7, wherein the emulsifier
is selected from the group consisting essentially of
phosphatidylcholine, lecithin, and combinations thereof.
9. The superabsorbent material of claim 5, further comprising a
surfactant in combination with the superabsorbent material.
10. The superabsorbent material of claim 9, wherein the surfactant
is selected from the group consisting essentially of sorbitan
monolaurate, compounds of the Triton series, compounds of the Brij
series, polyoxyethylene sorbitan monolaurate, polyoxyethylene
sorbitan tetraoleate, alcohol amines, and combinations thereof.
11. The superabsorbent material of claim 1, further comprising a
structure selected from the group consisting essentially of
particles, fibers, flakes, spheres, and combinations thereof.
12. A superabsorbent material, comprising: a water swellable, water
insoluble superabsorbent material; and, the superabsorbent material
having a first gel-bed friction angle at a superabsorbent material
swelling level of about 2.0 grams of 0.9 weight percent sodium
chloride solution/gram of the superabsorbent material and gel-bed
friction angles, at superabsorbent material swelling levels greater
than about 2.0 grams of 0.9 weight percent sodium chloride
solution/gram of the superabsorbent material, greater than the
first gel-bed friction angle, wherein the first gel-bed friction
angle is about 20 degrees or less.
13. The superabsorbent material of claim 12, wherein the first
gel-bed friction angle is 10 degrees or less.
14. The superabsorbent material of claim 12, further comprising a
friction angle increasing additive within the superabsorbent
material in combination with the water swellable, water insoluble
superabsorbent material.
15. The superabsorbent material of claim 14, wherein the friction
angle increasing additive is selected from the group consisting
essentially of chitosan, sodium silicate, sodium aluminate, alumino
silicates, and combinations thereof.
16. The superabsorbent material of claim 12, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of natural materials, modified
natural materials, synthetic materials, and combinations
thereof.
17. The superabsorbent material of claim 16, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of silica gels, agar, pectin, guar
gum, alkali metal salts of polyacrylic acids, polyacrylamides,
polyvinyl alcohols, ethylene maleic anhydride copolymers, polyvinyl
ethers, hydroxypropylcelluloses, polyvinyl morpholinones, polymers
and copolymers of vinyl sulfonic acid, polyacrylates,
polyacrylamides, polyvinyl pyridine, acrylonitrile grafted starch,
acrylic acid grafted starch, isobutylene maleic anhydride
copolymers, polyamines, and combinations thereof.
18. The superabsorbent material of claim 12, further comprising a
structure selected from the group consisting essentially of
particles, fibers, flakes, spheres, and combinations thereof.
19. An absorbent composite, comprising: a plurality of wettable
fibers; and, a water swellable, water insoluble superabsorbent
material in combination with the wettable fibers and having a first
gel-bed friction angle at a superabsorbent material swelling level
of about 2.0 grams of 0.9 weight percent sodium chloride
solution/gram of the superabsorbent material and gel-bed friction
angles, at superabsorbent material swelling levels greater than
about 2.0 grams of 0.9 weight percent sodium chloride solution/gram
of the superabsorbent material, substantially equal to or less than
the first gel-bed friction angle, wherein the first gel-bed
friction angle is about 20 degrees or less.
20. The absorbent composite of claim 19, wherein the first gel-bed
friction angle is about 10 degrees or less.
21. The absorbent composite of claim 19, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of natural materials, modified
natural materials, synthetic materials, and combinations
thereof.
22. The absorbent composite of claim 21, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of silica gels, agar, pectin, guar
gum, alkali metal salts of polyacrylic acids, polyacrylamides,
polyvinyl alcohols, ethylene maleic anhydride copolymers, polyvinyl
ethers, hydroxypropylcelluloses, polyvinyl morpholinones, polymers
and copolymers of vinyl sulfonic acid, polyacrylates,
polyacrylamides, polyvinyl pyridine, acrylonitrile grafted starch,
acrylic acid grafted starch, isobutylene maleic anhydride
copolymers, polyamines, and combinations thereof.
23. The absorbent composite of claim 19, further comprising a
friction angle reducing additive in combination with the
superabsorbent material.
24. The absorbent composite of claim 23, wherein the friction angle
reducing additive is selected from the group consisting essentially
of glycerol, mineral oil, silicone oil, polysaccharides,
polyethylene oxides, and combinations thereof.
25. The absorbent composite of claim 23, further comprising an
emulsifier in combination with the superabsorbent material.
26. The absorbent composite of claim 25, wherein the emulsifier is
selected from the group consisting essentially of
phosphatidylcholine, lecithin, and combinations thereof.
27. The absorbent composite of claim 23, further comprising a
surfactant in combination with the superabsorbent material.
28. The absorbent composite of claim 27, wherein the surfactant is
selected from the group consisting essentially of sorbitan
monolaurate, compounds of the Triton series, compounds of the Brij
series, polyoxyethylene sorbitan monolaurate, polyoxyethylene
sorbitan tetraoleate, alcohol amines, and combinations thereof.
29. The absorbent composite of claim 19, wherein the superabsorbent
material further comprises a structure selected from the group
consisting essentially of particles, fibers, flakes, spheres, and
combinations thereof.
30. The absorbent composite of claim 19, wherein the plurality of
wettable fibers is selected from the group consisting essentially
of natural fibers, synthetic fibers, and combinations thereof.
31. An absorbent composite, comprising: a plurality of wettable
fibers; and, a water swellable, water insoluble superabsorbent
material in combination with the wettable fibers and having a first
gel-bed friction angle at a superabsorbent material swelling level
of about 2.0 grams of 0.9 weight percent sodium chloride
solution/gram of the superabsorbent material and gel-bed friction
angles, at superabsorbent material swelling levels greater than
about 2.0 grams of 0.9 weight percent sodium chloride solution/gram
of the superabsorbent material, greater than the first gel-bed
friction angle, wherein the first gel-bed friction angle is about
20 degrees or less.
32. The absorbent composite of claim 31, wherein the first gel-bed
friction angle is about 10 degrees or less.
33. The absorbent composite of claim 31, further comprising a
friction angle increasing additive in combination with the water
swellable, water insoluble superabsorbent material.
34. The absorbent composite of claim 31, further comprising a
friction angle increasing additive in combination with the wettable
fibers.
35. The absorbent composite of claim 33, wherein the friction angle
increasing additive is selected from the group consisting
essentially of chitosan, sodium silicate, sodium aluminate, alumino
silicates, and combinations thereof.
36. The absorbent composite of claim 31, wherein the plurality of
wettable fibers is selected from the group consisting essentially
of natural fibers, synthetic fibers, and combinations thereof.
37. The absorbent composite of claim 31, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of natural materials, modified
natural materials, synthetic materials, and combinations
thereof.
38. The absorbent composite of claim 37, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of silica gels, agar, pectin, guar
gum, alkali metal salts of polyacrylic acids, polyacrylamides,
polyvinyl alcohols, ethylene maleic anhydride copolymers; polyvinyl
ethers, hydroxypropylcelluloses, polyvinyl morpholinones, polymers
and copolymers of vinyl sulfonic acid, polyacrylates,
polyacrylamides, polyvinyl pyridine, acrylonitrile grafted starch,
acrylic acid grafted starch, isobutylene maleic anhydride
copolymers, polyamines, and combinations thereof.
39. A superabsorbent material, comprising: a water swellable, water
insoluble superabsorbent material; and, the superabsorbent material
having a first gel-bed friction angle at a first superabsorbent
material swelling level of the superabsorbent material and gel-bed
friction angles, at superabsorbent material swelling levels greater
than the first superabsorbent material swelling level of the
superabsorbent material, greater than the first gel-bed friction
angle, wherein the first gel-bed friction angle is about 20 degrees
or less.
40. The superabsorbent material of claim 39, wherein the first
gel-bed friction angle is 10 degrees or less.
41. The superabsorbent material of claim 39, further comprising a
friction angle increasing additive within the superabsorbent
material in combination with the water swellable, water insoluble
superabsorbent material.
42. The superabsorbent material of claim 41, wherein the friction
angle increasing additive is selected from the group consisting
essentially of chitosan, sodium silicate, sodium aluminate, alumino
silicates, and combinations thereof.
43. The superabsorbent material of claim 39, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of natural materials, modified
natural materials, synthetic materials, and combinations
thereof.
44. The superabsorbent material of claim 43, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of silica gels, agar, pectin, guar
gum, alkali metal salts of polyacrylic acids, polyacrylamides,
polyvinyl alcohols, ethylene maleic anhydride copolymers, polyvinyl
ethers, hydroxypropylcelluloses, polyvinyl morpholinones, polymers
and copolymers of vinyl sulfonic acid, polyacrylates,
polyacrylamides, polyvinyl pyridine, acrylonitrile grafted starch,
acrylic acid grafted starch, isobutylene maleic anhydride
copolymers, polyamines, and combinations thereof.
45. The superabsorbent material of claim 39, further comprising a
structure selected from the group consisting essentially of
particles, fibers, flakes, spheres, and combinations thereof.
46. An absorbent composite, comprising: a plurality of wettable
fibers; and, a water swellable, water insoluble superabsorbent
material in combination with the wettable fibers and having a first
gel-bed friction angle at a first superabsorbent material swelling
level of the superabsorbent material and gel-bed friction angles,
at superabsorbent material swelling levels greater than the first
superabsorbent material swelling level of the superabsorbent
material, greater than the first gel-bed friction angle, wherein
the first gel-bed friction angle is about 20 degrees or less.
47. The absorbent composite of claim 46, wherein the first gel-bed
friction angle is about 10 degrees or less.
48. The absorbent composite of claim 46, further comprising a
friction angle increasing additive in combination with the water
swellable, water insoluble superabsorbent material.
49. The absorbent composite of claim 46, further comprising a
friction angle increasing additive in combination with the wettable
fibers.
50. The absorbent composite of claim 48, wherein the friction angle
increasing additive is selected from the group consisting
essentially of chitosan, sodium silicate, sodium aluminate, alumino
silicates, and combinations thereof.
51. The absorbent composite of claim 46, wherein the plurality of
wettable fibers is selected from the group consisting essentially
of natural fibers, synthetic fibers, and combinations thereof.
52. The absorbent composite of claim 46, wherein the water
swellable, water insoluble superabsorbent material is selected from
the group consisting essentially of natural materials, modified
natural materials, synthetic materials, and combinations thereof.
Description
BACKGROUND
People rely on absorbent articles in their daily lives.
Absorbent articles, including adult incontinence articles, feminine
care articles, and diapers, are generally manufactured by combining
a substantially liquid-permeable topsheet; a substantially
liquid-impermeable backsheet attached to the topsheet; and an
absorbent core located between the topsheet and the backsheet. When
the article is worn, the liquid-permeable topsheet is positioned
next to the body of the wearer. The topsheet allows passage of
bodily fluids into the absorbent core. The liquid-impermeable
backsheet helps prevent leakage of fluids held in the absorbent
core. The absorbent core is designed to have desirable physical
properties, e.g. a high absorbent capacity and high absorption
rate, so that bodily fluids may be transported from the skin of the
wearer into the disposable absorbent article.
The present invention relates to water swellable, water insoluble
superabsorbent materials, which are often employed in an absorbent
core (also referred to as an absorbent composite), in part to help
"lock up" fluids entering the core. More specifically, the present
invention pertains to superabsorbent materials having a modified
friction angle measured in a gel-bed of the superabsorbent
material. The gel-bed friction angle of the superabsorbent
materials of the present invention is controllable and follows a
predetermined pattern. The present invention also relates to use of
the controlled gel-bed friction angle superabsorbent materials in
absorbent composites and absorbent articles incorporating such
absorbent composites. Controlling the gel-bed friction angle of the
superabsorbent materials may allow control of phenomena including,
but not limited to: the swelling of the superabsorbent material,
stresses experienced by the superabsorbent material and/or other
ingredients (e.g., fibers) in an absorbent composite; the
permeability of an absorbent composite containing the
superabsorbent material; and/or, the absorbency, resiliency, and
porosity of the absorbent composite. The present invention relates
to treatments for superabsorbent materials to manipulate gel-bed
friction angle and new superabsorbent materials having the desired
gel-bed friction angle characteristics.
Absorbent composites used in absorbent articles typically consist
of an absorbent material, such as a superabsorbent material, mixed
with a composite matrix containing natural and/or synthetic fibers.
As fluids enter the absorbent composite, the superabsorbent
material swells as it absorbs the fluids. The superabsorbent
material contacts the surrounding matrix components and possibly
other superabsorbent material as it swells. The full swelling
capacity of the superabsorbent material may be reduced due to
stresses acting on the superabsorbent materials (e.g., stresses
imposed by the matrix on superabsorbent material; external stresses
acting on the absorbent composite that comprises a matrix and
superabsorbent material, including, for example, stresses imposed
on an absorbent composite by a wearer during use; stresses imposed
by one portion of the superabsorbent material on another portion of
the superabsorbent material, whether directly or indirectly; etc.).
Furthermore, stresses acting on an absorbent composite comprising
the superabsorbent material may act to reduce interstitial pore
volume, i.e., space between superabsorbent material, fibers, other
ingredients, or some combination thereof (without being bound to a
particular analogy, and for purposes of explanation only, think of
a force acting on some unit area of a sponge-like material with
pores, with the force per unit area--i.e., stress--acting to reduce
the thickness of the sponge-like material, and, therefore, the
volume of the pores).
As the superabsorbent material swells, it may rearrange into void
spaces of the absorbent composite matrix as well as expand readily
against the matrix to create additional void space. Also, as the
superabsorbent material swells, stresses acting within and/or on
the absorbent composite may increase due--at least in part--to
expansion of the superabsorbent material, thereby reducing the pore
volume between: fibers, superabsorbent material, other ingredients
in the absorbent composite, or some combination there of. The
ability to rearrange within the composite matrix, and the magnitude
and extent of the stresses acting within and on the composite
matrix, depend on several factors specifically including a gel-bed
friction angle of the superabsorbent material. In addition, as the
superabsorbent material moves within the composite matrix, the
superabsorbent material may contact the components, such as fibers
and binding materials, of the surrounding matrix. Thus, the
frictional properties of the superabsorbent material may influence
the ability of the material to swell and rearrange or move within
the matrix, as well as the magnitude and extent of the stresses
acting within and on the composite matrix.
It is often desired that the superabsorbent material be able to
rotate and translate within the voids of the absorbent composite to
allow the superabsorbent material to swell as close to full
swelling capacity as is possible within the matrix. There is a need
for a superabsorbent material which may more easily rearrange
within the void space of the absorbent composite matrix. There is a
need for a way to control the physical mechanics that: allow the
superabsorbent material to rearrange within the absorbent composite
matrix; reduce or minimize the stresses acting within or on the
absorbent composite or its ingredient(s); and/or reduce the
reduction in pore volume that may accompany the build up of said
stresses.
SUMMARY
We have discovered that superabsorbent materials having controlled
gel-bed friction angles meet one or more of these needs.
Accordingly, the present invention is directed to superabsorbent
materials having controlled gel-bed friction angles. The
superabsorbent materials of the present invention have gel-bed
friction angles that follow controlled gel-bed friction angle
patterns substantially different than gel-bed friction angle
patterns followed by conventional superabsorbent materials. The
superabsorbent materials of the present invention may be produced
using non-conventional manufacturing processes to obtain desired
gel-bed friction angles or by treating with friction angle
increasing additives and/or friction angle reducing additives to
increase, decrease, or otherwise control the friction angle of the
superabsorbent gel-bed during swelling. Gel-bed friction angle is a
property of a gel-bed or superabsorbent material coming from
Mohr-Coulomb failure theory. A lower friction angle implies lower
inter-particle friction.
The superabsorbent material of the present invention may be a water
swellable, water insoluble superabsorbent material. The water
swellable, water insoluble superabsorbent material may have a first
gel-bed friction angle at a superabsorbent material swelling level
of about 2.0 grams of 0.9 weight percent sodium chloride
solution/gram of the superabsorbent material and gel-bed friction
angles, at superabsorbent material swelling levels greater than
about 2.0 grams of 0.9 weight percent sodium chloride solution/gram
of the superabsorbent material, substantially equal to or less than
the first gel-bed friction angle. The first gel-bed friction angle
may be about 20 degrees or less. In other embodiments, the gel-bed
friction angles may be greater than the first gel-bed friction
angle. The superabsorbent material of the present invention may be
utilized in an absorbent composite further comprising a plurality
of wettable fibers.
These and other features, aspects, and advantages of the present
invention will become better understood with regard to the
following description, appended claims, and accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS OF EXAMPLES AND/OR REPRESENTATIVE
EMBODIMENTS
FIG. 1 shows an example of a response of a porous medium to a
stress (i.e., a force per unit area) acting on the medium.
FIG. 2 shows an example of the state of stress of an arbitrary
element at equilibrium in a porous medium.
FIG. 3 shows an example of an arbitrary element and the normal
forces and shear forces acting on a plane passing through the
arbitrary element.
FIG. 4 shows an example of a Mohr Circle on a plot of shear stress
(y axis) versus normal stress (x axis).
FIG. 5 shows an example of a sequence of Mohr Circles corresponding
to one possible stress path on a plot of shear stress (y axis)
versus normal stress (x axis).
FIG. 6 shows an example of Mohr Circles in relation to a
Mohr-Coulomb failure envelope on a plot of shear stress (y axis)
versus normal stress (x axis).
FIG. 7 shows a specific example of Mohr Circles in relation to a
Mohr-Coulomb failure envelope on a plot of shear stress (y axis)
versus normal stress (x axis).
FIG. 8 shows an example of a friction-angle measuring device, in
this case a Jenike-Schulze Ring-Shear Tester, available in the U.S.
from Jenike & Johanson, Inc., a business having offices in
Westford, Mass.
DEFINITIONS
Within the context of this specification, each term or phrase below
will include the following meaning or meanings.
"Absorbency Under Load" (AUL) refers to the measure of the liquid
retention capacity of a material under mechanical load. It is
determined by a test which measures the amount, in grams, of a 0.9%
by weight aqueous sodium chloride solution a gram of material may
absorb in 1 hour under an applied load or restraining pressure of
about 0.3 pound per square inch (2,000 Pascals). A procedure for
determining AUL is provided in U.S. Pat. No. 5,601,542, which is
incorporated by reference in its entirety in a manner consistent
herewith.
"Absorbent article" includes, without limitation, diapers, training
pants, swim wear, absorbent underpants, baby wipes, incontinence
products, feminine hygiene products and medical absorbent products
(for example, absorbent medical garments, underpads, bandages,
drapes, and medical wipes).
"Fiber" and "Fibrous Matrix" includes, but is not limited to
natural fibers, synthetic fibers and combinations thereof. Examples
of natural fibers include cellulosic fibers (e.g., wood pulp
fibers), cotton fibers, wool fibers, silk fibers and the like, as
well as combinations thereof. Synthetic fibers can include rayon
fibers, glass fibers, polyolefin fibers, polyester fibers,
polyamide fibers, polypropylene. As used herein, it is understood
that the term "fibrous matrix" includes a plurality of fibers.
"Free Swell Capacity" refers to the result of a test which measures
the amount in grams of an aqueous 0.9% by weight sodium chloride
solution that a gram of material may absorb in 1 hour under
negligible applied load.
"Gel-bed friction angle" refers to the friction angle of a
superabsorbent material in a gel-bed as measured with a
Jenike-Shulze ring shear tester or other friction angle measuring
technique.
"Gradient" refers to a graded change in the magnitude of a physical
quantity, such as the quantity of superabsorbent material present
in various locations of an absorbent pad, or other pad
characteristics such as mass, density, or the like.
"Gel-bed" refers to an amount of superabsorbent material within a
container such as a ring shear cell.
"Homogeneously mixed" refers to the uniform mixing of two or more
substances within a composition, such that the magnitude of a
physical quantity of each of the substances remains substantially
consistent throughout the composition.
"Incontinence products" includes, without limitation, absorbent
underwear for children, absorbent garments for children or young
adults with special needs such as autistic children or others with
bladder/bowel control problems as a result of physical
disabilities, as well as absorbent garments for incontinent older
adults.
"Meltblown fiber" means fibers formed by extruding a molten
thermoplastic material through a plurality of fine, usually
circular, die capillaries as molten threads or filaments into
converging high velocity heated gas (e.g., air) streams which
attenuate the filaments of molten thermoplastic material to reduce
their diameter, which may be to microfiber diameter. Thereafter,
the meltblown fibers are carried by the high velocity gas stream
and are deposited on a collecting surface to form a web of randomly
dispersed meltblown fibers. Such a process is disclosed for
example, in U.S. Pat. No. 3,849,241 to Butin et al. Meltblown
fibers are microfibers which may be continuous or discontinuous,
are generally smaller than about 0.6 denier, and are generally self
bonding when deposited onto a collecting surface. Meltblown fibers
used in the present invention are suitably substantially continuous
in length. "Mohr circle" refers to a graphical representation of
the state of stress within a material subjected to one or more
forces. Mohr circles are described in more detail below.
"Mohr failure envelope" refers to the failure shear stress at the
failure plane as a function of the normal stress on that failure or
shear plane. Mohr failure envelopes are described in more detail
below.
"Polymers" include, but are not limited to, homopolymers,
copolymers, such as for example, block, graft, random and
alternating copolymers, terpolymers, etc. and blends and
modifications thereof. Furthermore, unless otherwise specifically
limited, the term "polymer" shall include all possible geometrical
configurations of the material. These configurations include, but
are not limited to isotactic, syndiotactic and atactic
symmetries.
"Superabsorbent" or "superabsorbent material" refers to a
water-swellable, water-insoluble organic or inorganic material
capable, under the most favorable conditions, of absorbing at least
about 10 times its weight and, more particularly, at least about 20
times its weight in an aqueous solution containing 0.9 weight
percent sodium chloride. The superabsorbent materials may be
natural, synthetic and modified natural polymers and materials. In
addition, the superabsorbent materials may be inorganic materials,
such as silica gels, or organic compounds such as cross-linked
polymers. The superabsorbent materials of the present invention may
embody various structure configurations including particles,
fibers, flakes, and spheres.
"Pattern" or "predetermined pattern" when mentioned in context with
gel-bed friction angle refers to a particular dependence of the
gel-bed friction angle on the swelling level of the superabsorbent
material. The pattern of the gel-bed friction angle may refer to
the changes in the gel-bed friction angle of a superabsorbent
material as a function of the swelling level of the superabsorbent
material.
"Spunbonded fiber" refers to small diameter fibers which are formed
by extruding molten thermoplastic material as filaments from a
plurality of fine capillaries of a spinnerette having a circular or
other configuration, with the diameter of the extruded filaments
then being rapidly reduced as by, for example, in U.S. Pat. No.
4,340,563 to Appel et al.; U.S. Pat. No. 3,692,618 to Dorschner et
al.; U.S. Pat. No. 3,802,817 to Matsuki et al.; U.S. Pat. Nos.
3,338,992 and 3,341,394 to Kinney; U.S. Pat. No. 3,502,763 to
Hartmann; U.S. Pat. No. 3,502,538 to Petersen; and, U.S. Pat. No.
3,542,615 to Dobo et al., each of which is incorporated by
reference in its entirety in a manner consistent herewith. Spunbond
fibers are quenched and generally not tacky when they are deposited
onto a collecting surface. Spunbond fibers are generally continuous
and often have average deniers larger than about 0.3, more
particularly, between about 0.6 and 10.
These terms may be defined with additional language in the
remaining portions of the specification.
Overview of Continuum Mechanics, Mohr Circles, and Mohr-Coulomb
Failure Theory
Given that our discovery is described using tools and terminology
from mechanics, an overview of continuum mechanics, Mohr circles,
and Mohr-Coulomb failure theory is provided for convenience. It
should be understood that this overview is for purposes of
explanation only--it provides an analytic framework for
characterizing the present invention, and should not be viewed as
limiting the present invention disclosed herein.
Absorbent articles and composites are porous by nature. The open
space between the various ingredients that make up the composite
(e.g., superabsorbent material and fibers) is commonly referred to
as void space or pore space. Pore space acts to store liquids
and/or provide a conduit or pathway for transporting liquid
throughout the absorbent composite or article. The volume of pore
space per unit volume of absorbent composite is commonly referred
to as "porosity." Generally absorbency performance is improved by
increasing porosity. For example, permeability of an absorbent
composite--i.e., the ability of the composite to facilitate liquid
transport--increases with increasing porosity (other factors, such
as specific surface area and tortuosity, being equal).
The application of a stress to a porous medium, such as an
absorbent composite or article, is known to cause a volumetric
deformation of the medium as a whole, as well as shear deformation
in the case of anisotropic stresses. FIG. 1 depicts an example of a
volumetric deformation of a porous medium. The left-most image of
FIG. 1 is labeled "Higher Porosity" 10 and shows a porous medium 12
without a weight applied to the uppermost planar surface 14 of the
porous medium 12 (with the uppermost planar area having some
discrete area): The right-most image of FIG. 1 is labeled "Lower
Porosity" 16 and shows the same porous medium 12' with a weight 18
applied to the uppermost planar surface 14' of the porous medium
12'. In response to the placement of the weight 18, which produces
a stress, or normal force per unit area, .sigma. 20, the thickness
decreases (as denoted by .DELTA. L 22). (Note: for purposes of the
present invention, compressive stresses are represented as having
positive values.)
For a porous medium 12 made up of individual ingredients such as
superabsorbent particles and fibers (e.g., an absorbent composite),
the thickness change of the porous medium 12 as a whole, .DELTA. L
22, likely does not result from a reduction in the individual
dimensions of individual particles and fibers (reductions in these
individual thicknesses would likely be small or negligible).
Instead, the decrease in the thickness of the porous medium 12 as a
whole, .DELTA. L 22, results from a reduction in porosity (or,
analogously, void volume). Accordingly, in the example depicted in
FIG. 1, an increase in stress, or normal force per unit area,
.sigma. 22, reduces the thickness .DELTA. L 22 of the porous medium
12 as a whole, and reduces the porosi fluid in the pores is a
compressible gas, then a normal stress acting on the surface of the
porous medium 12 would: compress the gas within the pores; or cause
a portion of the gas within the pores to exit the porous medium 12;
or, some combination thereof. If, in this same FIG. 1, a fluid in
the pores is an incompressible liquid, then a normal stress acting
on the surface of the porous medium 12 would cause a portion of the
liquid to exit the porous medium 12.)
The porous medium 12 of FIG. 1 may be examined further to analyze
the stresses acting on an arbitrary element within the porous
medium 12. FIG. 2 illustrates the state of stress of an arbitrary
element 30--here represented by the face of a cube--at equilibrium
(the arbitrary element is within a porous medium 32 being subjected
to an external stress .sigma..sub.external 34). For present
purposes, the arbitrary element 30 within the porous medium 32 is
treated as a continuum. In FIG. 2, the state of stress is
represented by two normal components of stress, .sigma..sub.h 36
acting horizontally on a face of the cube and .sigma., 38 acting
vertically on another face of the cube, as well as a shear stress
.tau. 40. The normal components of stress 36 are perpendicular to
the faces of the arbitrary element 30, whereas the shear stresses
40 are parallel to the faces of the arbitrary element 10.
It should be noted that if the shear stresses 40 are zero (i.e.,
.tau.=0), then the two normal stresses 36 are referred to as
principal stresses. Furthermore, when .tau.=0, then the larger of
the two normal stresses 36 is called the major principal stress
while the other is called the minor principal stress. For the
present discussion, the two stresses are assumed to be principal
stresses, with .sigma..sub.h.gtoreq..sigma..sub.v.
There are generally at least two contributions to stress generation
that combine to produce principal stresses such as those identified
in FIG. 2. The first is an external stress 34, possibly
non-uniform, acting on the boundary of the porous medium 32. This
stress is transmitted throughout the porous medium 32 in accordance
with well known force-balance equations. The second contribution
arises due to swelling of components that make up the porous medium
32 (e.g., a superabsorbent material). For example, the swelling of
blocks, or elements, immediately adjacent to the arbitrary element
30 depicted in FIG. 2, will cause an "internally" generated stress
acting on or along the arbitrary element 30 as other elements
attempt to expand against it and each other.
As stated above, when the stresses acting on an arbitrary element
30, such as that depicted in FIG. 2, are principal stresses, there
are no shear stresses 40 acting on the faces of the arbitrary
element 30. There is, however, shear stress 40 acting on other
imaginary planes passing through the depicted arbitrary element
30--planes oriented at some angle .alpha. 50 away from horizontal,
0<.alpha.<90.degree., as shown in FIG. 3. FIG. 3 depicts a
major principal stress .sigma..sub.h 52 acting on a major principal
plane 54, and a minor principal stress .sigma..sub.v 56 acting on a
minor principal plane 58. A normal stress .sigma..sub.n.alpha. 60
and a shear stress .tau..sub..alpha. 62 act on the imaginary or
arbitrary plane 64 oriented at angle .alpha. 50 away from
horizontal.
Obtaining the shear and normal forces 62 and 60, respectively,
acting on the arbitrary plane 64 passing through the element 66
depicted in FIG. 3 is simplified by using the graphical approach of
the Mohr circle, as illustrated in FIG. 4. FIG. 4 shows a plot of
shear stress (y-axis) 70 as a function of normal stress (x-axis)
72. For purposes of the present discussion the principal stresses
are assumed to be known (e.g., by calculation or measurement). The
x-y coordinates of the minor principal stress .sigma..sub.v 74 and
the major principal stress .sigma..sub.h 76 lie on the x-axis
(i.e., where the shear stress .tau. 70 is equal to zero). A
semi-circle 78 is drawn such that the coordinates of the minor and
major principal stresses 74 and 76, respectively, correspond to the
end points of the arc defining the perimeter of the semi-circle 78.
The radius of this semi-circle 78 equals one-half of the difference
between the major principal stress .sigma..sub.h 76 and the minor
principal stress .sigma..sub.v 74. By constructing a radial line
segment 80 at an angle 2.alpha. 82 from the x-axis, with one end of
the radial line segment 80 corresponding to the center of the
semi-circle 78, and other end corresponding to a point on the
semi-circle arc closest to the major principal stress, both the
normal stress, .sigma..sub.n.alpha.84, and the shear stress
.tau..sub..alpha. 86 are obtained at the intersection 88 of the
radial line segment 80 with the Mohr semi-circle 78.
FIG. 5 depicts one example of stress evolution for a porous medium
that employs one or more swelling components (e.g., a particulate
superabsorbent material). The y-axis again corresponds to shear
stress .tau. 100, and the x-axis again corresponds to normal stress
.sigma. 102. If the minor principal stress .tau..sub.v 104 acting
on an arbitary element from the porous medium remains unchanged,
then stress development (which would accompany, for example,
swelling of superabsorbent material) may be viewed as a family of
Mohr circles 106, 108,110, and 112, all of which have the same
minor principal stress .sigma..sub.v 104. The progression of the
Mohr circles 106, 108, 110, and 112 is commonly referred to as a
stress path 114--more precisely, the line passing through the set
of the Mohr circles 106, 108, 110, and 112 at points simultaneously
locating the maximum shear stress and mean stress for each Mohr
circle 106, 108, 110, and 112.
The center of each Mohr circle 106,108, 110, and 112, which equates
to the mean stress, determines the extent of the volumetric
deformation of pore space contained within a particular arbitrary
element, and may correspond to the approximate stress experienced
by superabsorbent materials.
Stresses in a porous medium are not likely to increase
indefinitely--rather, failure will take place, accompanied by
sliding along particular failure planes (e.g., at the interface
between superabsorbent material and fiber; or at the interface
between individual particles of superabsorbent material; etc.). The
Mohr-Coulomb failure criterion states that a shear force acting on
a plane at failure will be linearly proportional to the normal
force acting on that same plane, again at failure. Hence,
Mohr-Coulomb theory provides a failure limit, or envelope, beyond
which stable states of stress do not exist. If a line corresponding
to this failure limit is superimposed on a plot of shear stress and
normal stress depicting a Mohr circle 106, 108, 110, and 112 (which
may be thought of as corresponding to a given state or degree of
swelling for a porous medium employing a superabsorbent material),
then the Mohr circle 106, 108, 110, and 112 may only increase in
radius (e.g., by additional swelling of the porous medium and/or
superabsorbent material employed by the porous medium) to the
extent that it becomes tangent to this linear envelope.
FIG. 6 depicts a linear failure envelope 120 on a plot of shear
stress .tau. 122 versus normal stress .sigma. 124. On this plot are
depicted each Mohr circle having a different value of initial
stress--that is, two different values of the minor principal stress
.sigma..sub.v 130 and 130'. The friction angle .phi. 132 and
cohesion c 134 are properties of a particular material (e.g., an
absorbent composite comprising fiber and superabsorbent material; a
gel bed of swollen, particulate superabsorbent material; etc.). The
tangent of the friction angle .phi. 132, which is equivalent to the
coefficient of static friction from elementary physics, measures
the extent to which an increasing normal force permits a larger
maximum shear force. Cohesion c 134 represents the amount of shear
stress a material will tolerate before failure in the absence of
any normal force on the proposed failure plane. An increase in any
one of the three parameters--friction angle .phi. 132, cohesion c
134, or minor principle stress .sigma..sub.v 130 and 130'--will
permit the development of larger stresses in a porous
material--i.e., a larger Mohr circle. Friction angle .phi. 132 and
cohesion c 134 are properties of the material and may be measured
(e.g., using the test and methodology disclosed herein). FIG. 6
also depicts the mathematical relationship
.tau..sub.ff=c+.sigma..sub.nff (tan .phi.) 136, which relates
friction angle .phi. 132, cohesion c 134, shear stress at failure
.tau..sub.ff 138, and normal stress at failure .sigma..sub.nff 140.
(Note: for purposes of this disclosure, .sigma..sub.nff is
equivalent to .sigma..sub.ff, with both terms referring to a normal
stress acting on the failure plane at failure.) This relationship
is described in more detail below in the Detailed Description
section.
As stated earlier, it is generally advantageous to minimize or
decrease the reduction of porosity, or void volume, that results
from the application of a compressive stress to an absorbent
article. By choosing materials that limit stress increases (e.g.,
low, controlled gel-bed friction angle superabsorbent material) the
magnitude of porosity reductions may be decreased. For example,
low, controlled gel-bed friction angle superabsorbent material will
promote the onset of failure before stresses rise to values that
cause significant losses of porosity, and therefore permeability.
An additional benefit of providing stress relief through low,
controlled gel-bed friction angle materials is that such
superabsorbent materials will retain a larger portion of their
free-swell capacity--since it is well known that superabsorbent
capacity decreases with increasing loading.
DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS
The present invention relates to water swellable, water insoluble
superabsorbent materials and the use of the superabsorbents in
absorbent composites of absorbent articles.
Absorbent composites of absorbent articles typically contain
superabsorbent material, in relatively high quantities in some
cases, in various forms such as superabsorbent fibers and/or
superabsorbent particles, homogeneously mixed with a matrix
material, such as cellulose fluff pulp. The mixture of
superabsorbent material and cellulose fluff pulp may be homogeneous
throughout the absorbent composite or the superabsorbent material
may be strategically located within the absorbent composite, such
as forming a gradient within the fiber matrix. For example, more
superabsorbent material may be present at one end of the absorbent
composite than at an opposite end of the absorbent composite.
Alternatively, more superabsorbent material may be present along a
top surface of the absorbent composite than along a bottom surface
of the absorbent composite or more superabsorbent material may be
present along the bottom surface of the absorbent composite than
along the top surface of the absorbent composite. One skilled in
the art will appreciate the various embodiments available for
absorbent composites. The water swellable, water insoluble
superabsorbent materials of the present invention may be used in
these and other various embodiments of absorbent composites.
Absorbent composites typically include a matrix which contains the
superabsorbent material. The matrix is often made from a fibrous
material or foam material, but one skilled in the art will
appreciate the various embodiments of the composite matrix. One
such fibrous matrix is made of a cellulose fluff pulp. The
cellulose fluff pulp suitably includes wood pulp fluff. The
cellulose pulp fluff may be exchanged, in whole or in part, with
synthetic, polymeric fibers (e.g., meltblown fibers). Synthetic
fibers are not required in the absorbent composites of the present
invention, but may be included. One preferred type of wood pulp
fluff is identified with the trade designation CR1654, available
from Bowater, Childersburg, Ala., U.S.A., and is a bleached, highly
absorbent wood pulp containing primarily soft wood fibers. The
cellulose fluff pulp may be homogeneously mixed with the
superabsorbent material. Within the absorbent article, the
homogeneously mixed fluff and superabsorbent material may be
selectively placed into desired zones of higher concentration to
better contain and absorb body exudates. For example, the mass of
the homogeneously mixed fluff and superabsorbent materials may be
controllably positioned such that more basis weight is present in a
front portion of the pad than in a back portion of the pad.
Absorbent composites of the present invention may suitably contain
between about 5 to about 95 mass % of superabsorbent material,
based on the total weight of the fiber, the superabsorbent
material, and/or any other component. Optionally, the mass
composition of the superabsorbent material in the absorbent
composite may be from about 20 to about 80%. Additionally, the mass
composition of the superabsorbent material in the absorbent
composite may be from about 40 to about 60%.
Suitable superabsorbent materials useful in the present invention
may be selected from natural, synthetic, and modified natural
polymers and materials. The superabsorbent materials may be
inorganic materials, such as silica gels, or organic compounds,
including natural materials such as agar, pectin, guar gum, and the
like, as well as synthetic materials, such as synthetic hydrogel
polymers. Such hydrogel polymers include, for example, alkali metal
salts of polyacrylic acids; polyacrylamides; polyvinyl alcohol;
ethylene maleic anhydride copolymers; polyvinyl ethers;
hydroxypropylcellulose; polyvinyl morpholinone; polymers and
copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides,
polyvinyl pyridine; polyamines; and, combinations thereof. Other
suitable polymers include hydrolyzed acrylonitrile grafted starch,
acrylic acid grafted starch, and isobutylene maleic anhydride
copolymers and combinations thereof. The hydrogel polymers are
suitably lightly crosslinked to render the material substantially
water-insoluble. Crosslinking may, for example, be by irradiation
or by covalent, ionic, Van der Waals, or hydrogen bonding. The
superabsorbent materials of the present invention may be in any
form suitable for use in absorbent structures, including,
particles, fibers, flakes, spheres, and the like.
Typically, a superabsorbent polymer is capable of absorbing at
least about 10 times its weight in a 0.9 weight percent aqueous
sodium chloride solution, and particularly is capable of absorbing
more than about 20 times its weight in 0.9 weight percent aqueous
sodium chloride solution. Superabsorbent polymers suitable for
treatment or modification in accordance with the present invention
are available from various commercial vendors, such as Dow Chemical
Company located in Midland, Mich., U.S.A., and Stockhausen Inc.,
Greensboro, N.C., USA. Other superabsorbent polymers suitable for
treatment or modification in accordance with the present invention
are described in U.S. Pat. No. 5,601,542 issued Feb. 11, 1997, to
Melius et al.; U.S. patent application Ser. No. 09/475,829 filed in
December 1999 and assigned to Kimberly-Clark Corporation; and, U.S.
patent application Ser. No. 09/475,830 filed in December 1999 and
assigned to Kimberly-Clark Corporation; each of which is hereby
incorporated by reference in a manner consistent herewith.
Other examples of commercial superabsorbent materials that may be
modified for use in the present invention include polyacrylate
materials available from Stockhausen under the tradename
FAVOR.RTM.. Examples include FAVOR.RTM. SXM 77, FAVOR.RTM. SXM 880,
and FAVOR.RTM. SXM 9543. Other polyacrylate superabsorbent
materials that may be modified for use in the present invention are
available from Dow Chemical, USA under the tradename DRYTECH.RTM.,
such as DRYTECH.RTM. 2035.
The superabsorbent materials of the present invention may be in the
form of particles which, in the unswollen state, have maximum
cross-sectional diameters typically within the range of from about
50 microns to about 1,000 microns, suitably within the range of
from about 100 microns to about 800 microns, as determined by sieve
analysis according to American Society for Testing Materials (ASTM)
Test Method D-1921. It is understood that the particles of
superabsorbent material, falling within the ranges described above,
may include solid particles, porous particles, or may be
agglomerated particles including many smaller particles
agglomerated into particles within the described size ranges.
Absorbent composites may also contain any of a variety of chemical
additives or treatments, fillers or other additives, such as clay,
zeolites and/or other odor-absorbing material, for example
activated carbon carrier particles or active particles such as
zeolites and activated carbon. Absorbent composites may also
include binding agents, such as crosslinkable binding agents or
adhesives, and/or binder fibers, such as bicomponent fibers.
Absorbent composites may or may not be wrapped or encompassed by a
suitable tissue wrap that maintains the integrity and/or shape of
the absorbent composite.
The structure and components of absorbent composites are designed
to take up fluids and absorb them. The porosity of the fiber matrix
allows fluid to penetrate the absorbent composite and contact the
superabsorbent material, which absorbs the fluids. The
superabsorbent material swells as the superabsorbent material
absorbs fluids. The swelling of the superabsorbent material may be
influenced by the external factors such as surrounding matrix
material and pressures (i.e., a force per unit area, or stress)
from the absorbent article user. The surrounding matrix fibers
and/or superabsorbent materials and the pressures on the
superabsorbent material may inhibit the swelling of the
superabsorbent material, thus stopping absorbency, and thereby the
absorbent composite, from reaching full free swell capacity. Also,
as described above, stresses acting on an absorbent composite, such
as an absorbent composite employing a superabsorbent material, may
reduce porosity and/or permeability of the absorbent composite.
To the extent possible during swelling, superabsorbent materials
may move within the composite matrix to positions that allow the
superabsorbent to obtain greater swelling. Superabsorbent materials
may rotate and/or translate so as to fit within voids in the
composite matrix which allows the absorbent particle to swell
readily against surrounding matrix and reach greater swelling
potentials. Moreover, additional voids/void space may be created by
overall expansion of the absorbent composite. Upon moving within
the fiber matrix, the superabsorbent materials will contact and rub
against other components of the absorbent composite, including
matrix fibers and/or other superabsorbent materials. The surface
mechanics of the superabsorbent material and the surrounding matrix
components may determine the amount of superabsorbent material
structure rotation and/or translation and thus may affect: (1) the
swelling capacity of the superabsorbent material, and therefore the
absorbent composite; and (2) the level of stress buildup in an
absorbent composite employing the superabsorbent, which in turn
affects the porosity and permeability of the absorbent
composite.
The friction angle of the superabsorbent material is an important
mechanical property that may affect the ability of the
superabsorbent material to move or expand within the absorbent
composite matrix. As discussed above in the Overview section,
friction angle comes from Mohr-Coulomb failure theory, and the
tangent of the friction angle is equivalent to the traditional
coefficient of static friction. A smaller friction angle may
indicate less contact friction between the superabsorbent material
and the surrounding matrix, and a greater ability for the
superabsorbent material to rearrange within the matrix during
swelling so that the superabsorbent material may retain a greater
portion of the free swell absorbent capacity. Also, a smaller
friction angle may promote failure (i.e., movement between, for
example, swollen particles of superabsorbent material; or movement
between a swollen particle of superabsorbent material and the
surrounding fiber matrix; etc.) at lower levels of stress buildup,
thereby reducing losses in porosity and/or permeability in an
absorbent composite.
The state of failure between the surfaces of the superabsorbent
material and the surrounding components allows the superabsorbent
material to rearrange within the wet matrix or a partially swollen
gel-bed. As indicated in the Overview Section, Mohr circles may be
used to describe the state of stress of a material, such as a wet
gel-bed or absorbent composite or porous medium. FIG. 7 shows
representative Mohr circles 150 and 152 for a typical gel-bed
swollen to a particular level. FIG. 7 shows Mohr circles 150 and
152 for the superabsorbent FAVOR.RTM. 9543 at a 2.0 grams saline
solution/gram superabsorbent material swelling level. The larger
Mohr circle 152 represents a situation where some pre-consolidation
stress is imposed on the gel-bed, and the smaller Mohr circle 150
represents the situation where some major principal stress exists
anywhere in the gel-bed while the minor principle stress is zero.
Although not shown in FIG. 7, Mohr circles are produced at each
applied normal stress. The state of failure for a superabsorbent
material is described by the set of Mohr circles at failure which
together define a Mohr failure envelope. The Mohr failure envelope
is often very close to linear, shown in FIG. 7 as line 154, and
represents the shear stress at failure, on the failure plane,
versus the normal stress acting on the same plane. The linearized
failure envelope 154, often referred to as the Mohr-Coulomb failure
criterion, may be represented mathematically by the formula:
.tau..sub.ff=c+.sigma..sub.ff(tan .phi.) where .tau..sub.ff is
shear stress, c is the effective cohesion constant, .sigma..sub.ff
is normal stress, and .phi. is the friction angle of the gel-bed or
superabsorbent material. The effective cohesion constant is
represented on the graph by value 156 and pertains to the cohesion
of the absorbent particle to the surrounding medium.
The gel-bed friction angle of the superabsorbent materials of the
present invention may be determined using various methods used in
fields such as soil mechanics. Useful instruments for determining
gel-bed friction angle include triaxial shear measurement
instruments, such as a Sigma1, available from GeoTac, Houston,
Tex., or ring shear testers such as the Jenike-Shulze Ring Shear
Tester, available from Jenike & Johanson, Westford, Mass.
FIG. 8 shows a partial cut-away schematic of a Jenike-Shulze Ring
Shear Tester, designated as reference numeral 170. The ring shear
tester 170 has a ring shear cell 172 connected to a motor (not
shown) that may rotate the ring shear cell 172 in direction co. The
ring shear cell 172 and lid 174 contain the superabsorbent material
gel-bed 176 to be tested. The lid 174 is not fixed to the ring
shear cell 172 and the crossbeam 178 crosses the lid 174 and
connects two guiding rollers 180 and two tie rods 182 to lid 174.
For measuring the gel-bed friction angle of swelled superabsorbent
material gel-bed 176 the superabsorbent material is swelled outside
the ring shear cell 172 and placed in the ring shear cell 172. A
predetermined force N may be placed upon the lid 174, and therefore
on the superabsorbent material 176, by a weight (not shown). A
counterweight system (not shown) may be engaged to test at lower
normal pressure. As the ring shear cell 172 rotates in direction
.omega. by the computer controlled motor (not shown), a shear
stress is placed on the superabsorbent material gel-bed 176
contacting the ring shear cell 172. An instrument connected to the
tie rods 182 measures the forces F1 and F2, which are used to
determine the shear stress at failure (for a given applied normal
stress) of the superabsorbent material gel-bed 176.
Superabsorbent material having a low gel-bed friction angle may be
useful in absorbent composites. In one embodiment of the present
invention, the superabsorbent material gel-bed friction angle
decreases upon swelling to about 20 degrees or less at a
superabsorbent material swelling level of about 2.0 grams of 0.9
weight percent aqueous sodium chloride solution/gram of
superabsorbent material (gram/gram) and remains at about 20 degrees
or less at swelling levels greater than 2.0 gram/gram. More
suitably the superabsorbent material gel-bed friction angle
decreases upon swelling to about 15 degrees or less at a
superabsorbent material swelling level of about 2.0 grams of 0.9
weight percent aqueous sodium chloride solution/gram of
superabsorbent material and remains at about 15 degrees or less at
swelling levels greater than 2.0 gram/gram. More particularly, the
superabsorbent material gel-bed friction angle decreases upon
swelling to about 10 degrees or less at a superabsorbent material
swelling level of about 2.0 grams of 0.9 weight percent aqueous
sodium chloride solution/gram of superabsorbent material, and
remains at about 10 degrees or less at swelling levels greater than
2.0 gram/gram
The low gel-bed friction angle superabsorbent materials of the
present invention reduce the local stresses between the
superabsorbent materials and/or the surrounding matrix components,
which may allow the superabsorbent material structures to rearrange
within the voids of an absorbent composite matrix more easily. The
low gel-bed friction angle superabsorbent materials may allow for
the superabsorbent materials to obtain a greater portion of their
free swell absorbent capacity. In addition, permeability is
generally maintained at suitable values because the development of
higher internal stresses is alleviated. As indicated above, the
buildup of stresses may result in additional compression of pore
space.
Low superabsorbent material gel-bed friction angles may be obtained
through non-conventional manufacturing processes that produce
superabsorbent material structures possessing low-friction surfaces
(e.g., smooth surfaces). Low superabsorbent material gel-bed
friction angles may also be obtained by treatment of superabsorbent
materials with friction angle reducing additives that decrease
friction angle upon becoming wet. Examples of such friction angle
reducing additives include, without limitation, glycerol, oils such
as mineral oil and silicone oil, oleic acid, polysaccharides,
polyethylene oxides.
The amount of gel-bed friction angle reducing additives,
surfactants, or emulsifiers may be about 1.0% by weight of the
swollen or unswollen superabsorbent material or less. Optionally,
the amount of gel-bed friction angle reducing additives,
surfactants, or emulsifiers may be about 10.0% by weight of the
swollen or unswollen superabsorbent material or less. Additionally,
the amount of gel-bed friction angle reducing additives,
surfactants, or emulsifiers may be about 100.0% by weight of the
swollen or unswollen superabsorbent material or less. The amount of
gel-bed friction angle reducing additives, surfactants, or
emulsifiers may be about 0.001% by weight of the swollen or
unswollen superabsorbent material or greater. Optionally, the
amount of gel-bed friction angle reducing additives, surfactants,
or emulsifiers may be about 0.1% by weight of the swollen or
unswollen superabsorbent material or greater. Additionally, the
amount of gel-bed friction angle reducing additives, surfactants,
or emulsifiers may be about 1.0% by weight of the swollen or
unswollen superabsorbent material or greater.
Small concentrations of emulsifiers and/or surfactants in addition
to the friction angle reducing additives, and friction angle
reducing additive mixtures such as a 50/50 by weight mixture of
glycerol and mineral oil, may help reduce the gel-bed friction
angle of the superabsorbent materials. The emulsifiers and
surfactants may increase the miscibility between nonpolar friction
angle reducing additives, such as mineral oil, and polar friction
angle reducing additives, such as glycerol. The emulsifiers and
surfactants may also play an integral role in coating the swollen
superabsorbent materials. Various emulsifiers and/or surfactants
may be used in the present invention depending on the friction
angle reducing additive used. Examples of emulsifiers are
phosphatidylcholine and lecithin. Examples of liquid surfactants
include sorbitan monolaurate, compounds of the TRITON.RTM. series
(X-100, X-405 & SP-135) available from J. T. Baker, compounds
of the BRIJ.RTM. series (92 and 97) available from J. T. Baker,
polyoxyethylene (80) sorbitan monolaurate, polyoxyethylene sorbitan
tetraoleate, and triethanolamine and other alcohol amines, and
combinations thereof. When using mixtures of polar and nonpolar
compounds, such as friction angle or cohesion value altering
additives, emulsifiers, and surfactants, the nonpolar compound may
be present in a larger proportion than the polar compound.
Absorbent composites of the present invention may include various
controlled gel-bed friction angle superabsorbent materials of the
present invention, including superabsorbent materials having low
gel-bed friction angles. The superabsorbent materials with
controlled gel-bed friction angles may be homogeneously mixed
within the absorbent composite or strategically located within
different absorbent composite areas, where the respective
controlled gel-bed friction angles are desired.
In one embodiment of the present invention, the gel-bed friction
angle of the superabsorbent material decreases upon swelling to a
first friction angle of about 20 degrees or less at a
superabsorbent material swelling level of about 2.0 grams of 0.9
weight percent aqueous sodium chloride solution/gram of
superabsorbent material, and the gel-bed friction angles may
increase as the swelling level increases. More suitably, the
gel-bed friction angle of the superabsorbent material decreases
upon swelling to a first friction angle of about 15 degrees or less
at a superabsorbent material swelling level of about 2.0 grams of
0.9 weight percent aqueous sodium chloride solution/gram of
superabsorbent material, and the gel-bed friction angles may
increase as the swelling level increases. More particularly, the
gel-bed friction angle of the superabsorbent material decreases
upon swelling to a first friction angle of about 10 degrees or less
at a superabsorbent material swelling level of about 2.0 grams of
0.9 weight percent aqueous sodium chloride solution/gram of
superabsorbent material, and the gel-bed friction angles may
increase as the swelling level increases.
Low superabsorbent material gel-bed friction angles at lower
swelling levels followed by high superabsorbent material gel-bed
friction angles at higher swelling levels combines the advantages
of the low gel-bed friction angles during the initial, early stages
of swelling, allowing for the desired failure and rearrangement of
the superabsorbent materials, with the advantages of the high
gel-bed friction angles, additional support for maintaining
composite integrity and permeability. Thus the superabsorbent
material may obtain more of its free swell capacity and maintain
desired absorbent composite porosity and permeability.
In one embodiment of the present invention, the gel-bed friction
angle of the superabsorbent material (specifically, a
superabsorbent material initially having a lower gel-bed friction
angle, such as one or more of the low gel-bed friction angle
superabsorbent materials described above) may be increased during
swelling with a friction angle increasing additive that is located
within the superabsorbent material structures in combination with
the water swellable, water insoluble polymer. In one embodiment of
the present invention, the friction angle increasing additive may
be chitosan, which may create a sticky condition between anionic
superabsorbent polymers, leading to a higher friction angle. Other
examples of such friction angle increasing additives include,
without limitation, sodium silicate, sodium aluminate, and alumino
silicates.
The amount of gel-bed friction angle increasing additives,
surfactants, or emulsifiers may be about 1.0% by weight of the
swollen or unswollen superabsorbent material or less. Optionally,
the amount of gel-bed friction angle increasing additives,
surfactants, or emulsifiers may be about 10.0% by weight of the
swollen or unswollen superabsorbent material or less. Additionally,
the amount of gel-bed friction angle increasing additives,
surfactants, or emulsifiers may be about 100.0% by weight of the
swollen or unswollen superabsorbent material or less. The amount of
gel-bed friction angle increasing additives, surfactants, or
emulsifiers may be about 0.001% by weight of the swollen or
unswollen superabsorbent material or greater. Optionally, the
amount of gel-bed friction angle increasing additives, surfactants,
or emulsifiers may be about 0.1% by weight of the swollen or
unswollen superabsorbent material or greater. Additionally, the
amount of gel-bed friction angle increasing additives, surfactants,
or emulsifiers may be about 1.0% by weight of the swollen or
unswollen superabsorbent material or greater.
The friction angle increasing additive may have a tendency to
migrate from within the polymer structure to the surface of the
superabsorbent material as the superabsorbent material swells. In
effect, the friction angle increasing additive may not coat, or
substantially coat, the superabsorbent material surface when dry
and, upon wetting, it migrates to the surface during swelling,
thereby causing the gel-bed friction angle of the superabsorbent
material to increase. The friction angle increasing additives may
be organic and/or inorganic additives, either natural or
synthetic.
Small concentrations of emulsifiers and/or surfactants may be used
in addition to the friction angle increasing additives, and
friction angle increasing additive mixtures, may help increase the
gel-bed friction angle of the superabsorbent materials. The
emulsifiers and surfactants may increase the miscibility between
nonpolar friction angle increasing additives and polar friction
angle increasing additives. The emulsifiers and surfactants may
also play an integral role in coating the swollen superabsorbent
materials. Various emulsifiers and/or surfactants may be used in
the present invention depending on the friction angle increasing
additive used. Examples of emulsifiers are phosphatidylcholine and
lecithin. Examples of liquid surfactants include sorbitan
monolaurate, compounds of the TRITON.RTM. series (X-100, X-405
& SP-135) available from J. T. Baker, compounds of the
BRIJ.RTM. series (92 and 97) available from J. T. Baker,
polyoxyethylene (80) sorbitan monolaurate, polyoxyethylene sorbitan
tetraoleate, and triethanolamine and other alcohol amines, and
combinations thereof.
In another embodiment of the present invention, the gel-bed
friction angle of the superabsorbent material (specifically, a
superabsorbent material initially having a lower gel-bed friction
angle, such as one or more of the low gel-bed friction angle
superabsorbent materials described above) may be increased with a
friction angle increasing additive located within the matrix of the
absorbent composite. The friction angle increasing additive is in
combination with a matrix component, such as coated onto the
wettable matrix fibers. The friction angle increasing additive has
a tendency to release from the fibers upon wetting and associate
with the surface of the superabsorbent material to increase the
gel-bed friction angle of the superabsorbent material. Suitably,
the friction angle increasing additive debonds with the matrix
component at a controlled rate upon wetting, and thereby gradually
increases the gel-bed friction angle of the superabsorbent material
over a desired time period. The friction angle increasing additives
may be organic and/or inorganic additives, natural and/or synthetic
materials.
The additives, such as the friction angle increasing additives and
friction angle reducing additives, which may alter the friction
angle of superabsorbent materials, may be delivered either directly
or indirectly to the superabsorbent. Direct delivery could occur
through release from the superabsorbent material itself while
indirect delivery could occur from fiber or some other component
positioned within or adjacent the superabsorbent material and/or
the absorbent composite. Furthermore, friction angle altering
additives may be delivered gradually over some time period through
release from any of the existing components present in the
absorbent composite or as the result of some chemical reaction
devised to release the friction angle altering additive at the most
desirable moment. For example, the friction angle altering additive
may be attached to the surface of the superabsorbent material or
embedded within its interior, or it may be loaded onto and/or into
some other component present in the absorbent composite, including
but not limited to the fibrous material. The friction angle
altering additive may be available immediately, leading to
immediate alteration of the friction angle, or because of a
chemical reaction or diffusion or some other mechanism, gradually
alter the friction angle in the desired manner at some desired
time.
It may be desirable to treat the superabsorbent material, the fiber
and/or fibrous matrix, and/or other components that may be used in
an absorbent composite with a friction angle altering additive,
such as the friction angle reducing additive, the friction angle
increasing additive and/or combinations thereof, to provide
materials having desired initial friction angles. The material
treated with the friction angle altering additive to provide a
desired initial friction angle may then be treated with additional
friction angle altering additives in accordance with the present
invention. The term "substantially" when used herein in regard with
friction angle, means within +/- one degree. The term
"substantially" when used herein in regard with cohesion value,
means within +/-100 Pascals.
The controlled gel-bed friction angle superabsorbent materials of
the present invention may be incorporated into absorbent composites
useful in absorbent articles. The various controlled gel-bed
friction angle superabsorbent materials of the present invention
may be used in various composite structures known in the art, such
as described above, including fibrous composites such as meltblown,
airlaid, and spunbond composites and foam composites. The
superabsorbent materials of the present invention may be formed in
various structures in absorbent composites, including particles,
flakes, fibers, and spheres.
In accordance with one embodiment of the present invention, a
superabsorbent material may comprise a water swellable, water
insoluble superabsorbent material. The superabsorbent material may
have a first gel-bed friction angle at a superabsorbent material
swelling level of about 2.0 grams of 0.9 weight percent sodium
chloride solution/gram of the superabsorbent material. The
superabsorbent material also may have gel-bed friction angles, at
superabsorbent material swelling levels greater than about 2.0
grams of 0.9 weight percent sodium chloride solution/gram of the
superabsorbent material, substantially equal to or less than the
first gel-bed friction angle. The first gel-bed friction angle may
be about 20 degrees or less.
In accordance with other aspects of the present invention, the
first gel-bed friction angle may be about 10 degrees or less. The
water swellable, water insoluble superabsorbent material may be
selected from the group consisting essentially of natural
materials, modified natural materials, synthetic materials, and
combinations thereof. The superabsorbent material may further
comprise a structure selected from the group consisting of
particles, fibers, flakes, spheres, and combinations thereof.
The water swellable, water insoluble superabsorbent material may be
selected from the group consisting essentially of natural
materials, modified natural materials, synthetic materials, and
combinations thereof. The water swellable, water insoluble
superabsorbent material may be selected from the group consisting
essentially of silica gels, agar, pectin, guar gum, alkali metal
salts of polyacrylic acids, polyacrylamides, polyvinyl alcohols,
ethylene maleic anhydride copolymers, polyvinyl ethers,
hydroxypropylcelluloses, polyvinyl morpholinones, polymers and
copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides,
polyvinyl pyridine, acrylonitrile grafted starch, acrylic acid
grafted starch, isobutylene maleic anhydride copolymers,
polyamines, and combinations thereof.
The present invention may further comprise a friction angle
reducing additive in combination with the superabsorbent material.
The friction angle reducing additive may be selected from the group
consisting essentially of glycerol, mineral oil, silicone oil,
polysaccharides, polyethylene oxides, and combinations thereof. The
superabsorbent material may further comprise an emulsifier in
combination with the superabsorbent material. The emulsifier may be
selected from the group consisting essentially of
phosphatidylcholine, lecithin, and combinations thereof. The
superabsorbent material may further comprise a surfactant in
combination with the superabsorbent material. The surfactant may be
selected from the group consisting essentially of sorbitan
monolaurate, compounds of the Triton series, compounds of the Brij
series, polyoxyethylene sorbitan monolaurate, polyoxyethylene
sorbitan tetraoleate, alcohol amines, and combinations thereof.
In accordance with another embodiment of the present invention, a
superabsorbent material may comprise a water swellable, water
insoluble superabsorbent material. The superabsorbent material may
have a first gel-bed friction angle at a superabsorbent material
swelling level of about 2.0 grams of 0.9 weight percent sodium
chloride solution/gram of the superabsorbent material. The
superabsorbent also may have gel-bed friction angles, at
superabsorbent material swelling levels greater than about 2.0
grams of 0.9 weight percent sodium chloride solution/gram of the
superabsorbent material, greater than the first gel-bed friction
angle. The first gel-bed friction angle may be about 20 degrees or
less.
In accordance with other aspects of the present invention, the
first gel-bed friction angle may be about 10 degrees or less. The
superabsorbent material may further comprise a friction angle
increasing additive within the superabsorbent material in
combination with the water swellable, water insoluble
superabsorbent material. The friction angle increasing additive is
selected from the group consisting essentially of chitosan, sodium
silicate, sodium aluminate, alumino silicates, and combinations
thereof.
The water swellable, water insoluble superabsorbent material may be
selected from the group consisting essentially of natural
materials, modified natural materials, synthetic materials, and
combinations thereof. The superabsorbent material may further
comprise a structure selected from the group consisting of
particles, fibers, flakes, spheres, and combinations thereof.
The water swellable, water insoluble superabsorbent material may be
selected from the group consisting essentially of silica gels,
agar, pectin, guar gum, alkali metal salts of polyacrylic acids,
polyacrylamides, polyvinyl alcohols, ethylene maleic anhydride
copolymers, polyvinyl ethers, hydroxypropylcelluloses, polyvinyl
morpholinones, polymers and copolymers of vinyl sulfonic acid,
polyacrylates, polyacrylamides, polyvinyl pyridine, acrylonitrile
grafted starch, acrylic acid grafted starch, isobutylene maleic
anhydride copolymers, polyamines, and combinations thereof.
In accordance with another embodiment of the present invention, an
absorbent composite may comprise a water swellable, water insoluble
superabsorbent material and a plurality of wettable fibers. The
water swellable, water insoluble superabsorbent material in
combination with the wettable fibers may have a first gel-bed
friction angle at a superabsorbent material swelling level of about
2.0 grams of 0.9 weight percent sodium chloride solution/gram of
the superabsorbent material. The superabsorbent material also may
have gel-bed friction angles at superabsorbent material swelling
levels greater than about 2.0 grams of 0.9 weight percent sodium
chloride solution/gram of the superabsorbent material,
substantially equal to or less than the first gel-bed friction
angle. The first gel-bed friction angle may be about 20 degrees or
less.
In accordance with other aspects of the present invention, the
first gel-bed friction angle may be about 10 degrees or less. The
superabsorbent material may further comprise a structure selected
from the group consisting of particles, fibers, flakes, spheres,
and combinations thereof.
The water swellable, water insoluble superabsorbent material may be
selected from the group consisting essentially of natural
materials, modified natural materials, synthetic materials, and
combinations thereof. The water swellable, water insoluble
superabsorbent material may be selected from the group consisting
essentially of silica gels, agar, pectin, guar gum, alkali metal
salts of polyacrylic acids, polyacrylamides, polyvinyl alcohols,
ethylene maleic anhydride copolymers, polyvinyl ethers,
hydroxypropylcelluloses, polyvinyl morpholinones, polymers and
copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides,
polyvinyl pyridine, acrylonitrile grafted starch, acrylic acid
grafted starch, isobutylene maleic anhydride copolymers,
polyamines, and combinations thereof.
The present invention may further comprise a friction angle
reducing additive in combination with the superabsorbent material.
The friction angle reducing additive may be selected from the group
consisting essentially of glycerol, mineral oil, silicone oil,
polysaccharides, polyethylene oxides, and combinations thereof. The
superabsorbent material may further comprise an emulsifier in
combination with the superabsorbent material. The emulsifier may be
selected from the group consisting essentially of
phosphatidylcholine, lecithin, and combinations thereof. The
superabsorbent material may further comprise a surfactant in
combination with the superabsorbent material. The surfactant may be
selected from the group consisting essentially of sorbitan
monolaurate, compounds of the Triton series, compounds of the Brij
series, polyoxyethylene sorbitan monolaurate, polyoxyethylene
sorbitan tetraoleate, alcohol amines, and combinations thereof.
The present invention may further comprise a friction angle
reducing additive in combination with the wettable fibers. The
wettable fibers may be selected from the group consisting
essentially of natural fibers, synthetic fibers, and combinations
thereof.
In accordance with another embodiment of the present invention, an
absorbent composite may comprise a water swellable, water insoluble
superabsorbent material and a plurality of wettable fibers. The
water swellable, water insoluble superabsorbent material in
combination with the wettable fibers may have a first gel-bed
friction angle at a superabsorbent material swelling level of about
2.0 grams of 0.9 weight percent sodium chloride solution/gram of
the superabsorbent material. The water swellable, water insoluble
superabsorbent material also may have gel-bed friction angles, at
superabsorbent material swelling levels greater than about 2.0
grams of 0.9 weight percent sodium chloride solution/gram of the
superabsorbent material, greater than the first gel-bed friction
angle. The first gel-bed friction angle may be about 20 degrees or
less. In the alternative, the first gel-bed friction angle may be
about 10 degrees or less.
The present invention may further comprise a friction angle
increasing additive in combination with the water swellable, water
insoluble superabsorbent material. In the alternative, the friction
angle increasing additive may be in combination with the wettable
fibers. The friction angle increasing additive may be selected from
the group consisting essentially of chitosan, sodium silicate,
sodium aluminate, alumino silicates, and combinations thereof. The
wettable fibers may be selected from the group consisting
essentially of natural fibers, synthetic fibers, and combinations
thereof.
The water swellable, water insoluble superabsorbent material is
selected from the group consisting essentially of natural
materials, modified natural materials, synthetic materials, and
combinations thereof. The water swellable, water insoluble
superabsorbent material may be selected from the group consisting
essentially of silica gels, agar, pectin, guar gum, alkali metal
salts of polyacrylic acids, polyacrylamides, polyvinyl alcohols,
ethylene maleic anhydride copolymers, polyvinyl ethers,
hydroxypropylcelluloses, polyvinyl morpholinones, polymers and
copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides,
polyvinyl pyridine, acrylonitrile grafted starch, acrylic acid
grafted starch, isobutylene maleic anhydride copolymers,
polyamines, and combinations thereof.
In accordance with another embodiment of the present invention, a
superabsorbent material may comprise a water swellable, water
insoluble superabsorbent material. The superabsorbent material may
have a first gel-bed friction angle at a first superabsorbent
material swelling level of the superabsorbent material and gel-bed
friction angles, at superabsorbent material swelling levels greater
than the first superabsorbent material swelling level of the
superabsorbent material. The gel-bed friction angles may be greater
than the first gel-bed friction angle. The first gel-bed friction
angle may be about 20 degrees or less. In the alternative, the
first gel-bed friction angle may be 10 degrees or less.
In accordance with other aspects of the present invention, the
superabsorbent material may further comprise a friction angle
increasing additive within the superabsorbent material in
combination with the water swellable, water insoluble
superabsorbent material. The friction angle increasing additive may
be selected from the group consisting essentially of chitosan,
sodium silicate, sodium aluminate, alumino silicates, and
combinations thereof. The water swellable, water insoluble
superabsorbent material may be selected from the group consisting
essentially of natural materials, modified natural materials,
synthetic materials, and combinations thereof.
The water swellable, water insoluble superabsorbent material may be
selected from the group consisting essentially of silica gels,
agar, pectin, guar gum, alkali metal salts of polyacrylic acids,
polyacrylamides, polyvinyl alcohols, ethylene maleic anhydride
copolymers, polyvinyl ethers, hydroxypropylcelluloses, polyvinyl
morpholinones, polymers and copolymers of vinyl sulfonic acid,
polyacrylates, polyacrylamides, polyvinyl pyridine, acrylonitrile
grafted starch, acrylic acid grafted starch, isobutylene maleic
anhydride copolymers, polyamines, and combinations thereof. The
superabsorbent material may further comprise a structure selected
from the group consisting essentially of particles, fibers, flakes,
spheres, and combinations thereof.
In accordance with another embodiment of the present invention, an
absorbent composite may comprise a plurality of wettable fibers and
a water swellable, water insoluble superabsorbent material in
combination with the wettable fibers. The water swellable, water
insoluble superabsorbent material may have a first gel-bed friction
angle at a first superabsorbent material swelling level of the
superabsorbent material and gel-bed friction angles, at
superabsorbent material swelling levels greater than the first
superabsorbent material swelling level of the superabsorbent
material, greater than the first gel-bed friction angle. The first
gel-bed friction angle may be about 20 degrees or less. In the
alternative, the first gel-bed friction angle may be about 10
degrees or less.
In accordance with other aspects of the present invention, the
absorbent composite may further comprise a friction angle
increasing additive in combination with the water swellable, water
insoluble superabsorbent material. The absorbent composite may
further comprise a friction angle increasing additive in
combination with the wettable fibers. The friction angle increasing
additive may be selected from the group consisting essentially of
chitosan, sodium silicate, sodium aluminate, alumino silicates, and
combinations thereof. The plurality of wettable fibers may be
selected from the group consisting essentially of natural fibers,
synthetic fibers, and combinations thereof. The water swellable,
water insoluble superabsorbent material may be selected from the
group consisting essentially of natural materials, modified natural
materials, synthetic materials, and combinations thereof.
Friction Angle Determination
A ring shear testing device such as a Jenike-Schulze Ring Shear
Tester apparatus may be used to determine a superabsorbent material
gel-bed friction angle. For testing, a sufficient amount (200-1000
grams) of swollen superabsorbent material (e.g., swollen 0-30 g/g
or more) is placed within the ring shear cell. For the samples
described below, the standard procedure for determining `yield
locus` as described in the manuals `RST-01.pc, RST-CONTROL` for the
Jenike-Shulze Ring shear tester was followed. The specific details
for the material preparation and test procedure are given
below:
The superabsorbent material is swollen to the desired level by 0.9
weight percent aqueous sodium chloride (such as that available from
Ricca Chemical Company, Arlington, Tex.) in a Kitchen Aid.TM.
blender (model #K5SS, 5 Quart); by first pouring a specific amount
of the solution (200-1000 grams) in the blender bowl (bowl
approximate volume: 5 quart) and then adding a predetermined
quantity (22-600 grams) of dry superabsorbent material while the
stirrer is slowly churning the fluid at the lowest speed setting
(setting range 1-10, where 1 is the lowest and 10 is the highest).
This is done so as to distribute the swelling solution uniformly to
all the superabsorbent material. When all solution is absorbed by
the superabsorbent material (absorption time: 0-30 minutes), the
bowl is removed from the blender, covered so as to prevent
evaporation and allowed to equilibrate for one hour so that the
fluid is distributed evenly throughout each particle. The sample is
manually mixed every fifteen minutes to ensure that no clumps are
formed.
TABLE-US-00001 Saline SAP SAP- Dry Weight Weight Total Weight
Amount for Capacity Fluid Needed Needed SAP-Fluid standard Ring-
(g/g) Ratio (grams) (grams) (grams) Cell (grams) 1 1:1 250 250 500
350-450 2 1:2 150 300 450 350-450 5 1:5 80 400 480 400-480 10 1:10
50 500 550 450-550 15 1:15 40 600 640 540-640 20 1:20 30 600 630
550-630
If a coating is applied to the superabsorbent material, the
appropriate coating additive is prepared separately, for example,
as described below. The equilibrated (time approximately: 1 hour)
and swollen superabsorbent material is coated evenly using a
Kitchen Aid.TM. blender by first introducing the swollen
superabsorbent material into the bowl, and then slowly adding the
coating additive (addition time: 1-30 minutes) while turning the
superabsorbent material in the bowl at the lowest speed setting
(setting range 1-10, where 1 is the lowest and 10 is the highest)
with the stirrer at all times. The coated superabsorbent material
is allowed to rest for 0-30 minutes with manual mixing every five
minutes to maintain equal distribution of treatment.
The gel-bed friction angle and effective cohesion measurements are
determined by using the Jenike-Schulze Ring Shear Tester apparatus.
The Jenike-Schulze Ring Shear Tester is used to obtain the gel-bed
friction angle values of superabsorbent material gel-beds at
various swelling levels. The Ring Shear Tester is operated and
calibrated according to the manufacturer's instructions provided. A
sample is loaded into the ring shear cell (Volume Ring
Cell--standard: 942.48 cm.sup.3) while ensuring the superabsorbent
gel-bed is distributed evenly (see above table). After one hour of
assumed equilibration with 0.9 weight percent sodium chloride
solution is achieved, the ring shear cell is filled with the bulk
superabsorbent material to be tested (see above table). Even
filling may be obtained by removing excess material with a spatula,
without compressing the superabsorbent material. The superabsorbent
material gel-bed is suitably flush with the top of the ring shear
cell. The weight of the filled ring shear cell (without the lid) is
determined on a mass balance and recorded. The samples described
below were tested by the ring shear tester control program
(RSTCTRL) for 1-2 hours. On request from RSTCTRL, the filled shear
cell is securely placed on the driving axle. The lid is placed on
the ring shear cell and positioned a few degrees counterclockwise
from the shear position; the ring shear tester pre-sets this start
position. The handle of the counterweight should be on the right
side of the crossbeam, and the hook on the crossbeam should be
facing the handle. On request from RSTCTRL, the counter weight and
the hanger are hooked to the central axis of the crossbeam. The tie
rods are attached on each side of the crossbeam, and the ring shear
cell is adjusted so that the tie rods are not stressed. The
RST-Control offers the possibility to adjust the shear cell with
arrow keys: .rarw..fwdarw., and using: .uparw..dwnarw. to stop when
positioned properly.
During the test procedure, the pressures at which the sample is
pre-sheared are read from a control file. In the sample tests
described below, the pre-shearing normal pressure is set at 3000
Pascals and the pre-sheared/pre-consolidated gel-bed is then
sheared to failure, to obtain the Mohr-Coulomb envelope, at a range
of normal pressures ranging from 500 Pascals to 2500 Pascals.
Pre-shearing precedes each shearing measurement. Thus, every
superabsorbent material gel-bed is sheared twice at any shearing
normal pressure in one experiment. Sometimes the equipment needs to
be run in semiautomatic mode and the data point is obtained
manually. After the samples below were completed, the results were
analyzed using RSV 95, Version 1.0; the software package included
with the ring shear tester.
EXAMPLES
To demonstrate aspects of the present invention, superabsorbent
material, designated as FAVOR.RTM. SXM 9543, available from
Stockhausen, Inc., a business having offices in Greensboro, N.C.,
was treated to reduce the gel-bed friction angle.
Control
The gel-bed friction angle of the superabsorbent material,
untreated FAVOR.RTM. SXM 9543, was measured as a control at various
swelling levels. The results are summarized in Table 1.
TABLE-US-00002 TABLE 1 Swelling level (gram/gram) 2 5 10 15 20
Gel-bed friction angle (degree) 23 15 12 11 12
As a comparison with the FAVOR.RTM. SXM 9543 control, the gel-bed
friction angle of the superabsorbent material DRYTECH.RTM. 2035 was
also measured at various swelling levels. DRYTECH.RTM. 2035 is
available from Dow Chemical Company, a business having offices in
Midland, Mich. The results are summarized in Table 2.
TABLE-US-00003 TABLE 2 Swelling level (gram/gram) 2 5 10 15 Gel-bed
friction angle (degree) 29 17 11 4
Sample 1
An amount of FAVOR.RTM. SXM 9543 was first swollen to a swelling
level of 2 grams of 0.9 weight percent aqueous sodium chloride
solution per gram of superabsorbent material (gram/gram), and
equilibrated for one hour, as described above. A coating of
glycerol, CAS 56-81-5 (99 percent minimum), available from J. T.
Baker, a business having offices in Phillipsburg, N.J., in the
ratio of 1.0 gram of additive per 2.0 grams of the swollen
superabsorbent material was applied to the superabsorbent material.
The gel-bed friction angle was measured as described above. The
gel-bed friction angle of Sample 2 at the given swelling level was
found to be 20 degrees and is summarized in Table 3.
Sample 2
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 gram of 0.9 weight percent of aqueous sodium
chloride solution per gram of superabsorbent material, and
equilibrated for one hour, as described above. A coating of mineral
oil, CAS 8012-95-1 (white mineral oil with Vitamin E as a
stabilizer), available from J. T. Baker, a business having offices
in Phillipsburg, N.J., in the ratio of 1.0 gram of additive per 2.0
grams of the swollen superabsorbent material was applied to the
superabsorbent material. The gel-bed friction angle was measured as
described above. The gel-bed friction angle of the coated
superabsorbent material at the given swelling level was found to be
6 degrees and is summarized in Table 3.
Sample 3
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 grams of 0.9 weight percent of aqueous sodium
chloride solution per gram of superabsorbent material, and
equilibrated for one hour, as described above. A coating of
silicone oil, CAS 63148-62-9 (density 0.963 gram/cubic centimeter),
available from Sigma Aldrich, a business having offices in St.
Louis, Mo., in the ratio of 1.0 gram of additive per 2.0 grams of
the swollen superabsorbent material was applied to the
superabsorbent material. The gel-bed friction angle was measured as
described above. The gel-bed friction angle of the coated
superabsorbent material at the given swelling level was found to be
17 degrees and is summarized in Table 3.
Sample 4
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 grams of 0.9 weight percent of aqueous sodium
chloride solution per gram of superabsorbent material, and
equilibrated for one hour, as described above. A coating, of 50
percent by weight of mineral oil (from Sample 2) and 50 percent by
weight of glycerol (from Sample 1), in the ratio of 1.0 gram of
additive coating per 2.0 grams of the swollen superabsorbent
material was applied to the superabsorbent material. The gel-bed
friction angle was measured as described above. The gel-bed
friction angle of the coated superabsorbent material at the given
swelling level was found to be 11 degrees and is summarized in
Table 3.
Sample 5
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 grams of 0.9 weight percent of aqueous sodium
chloride solution per gram of superabsorbent material, and
equilibrated for one hour, as described above. A coating, mineral
oil (from Sample 2), glycerol (from Sample 1), and lecithin, CAS
8002-43-5 (dry, granular), available from Spectrum Quality
Products, Inc., a business having offices in Gardena, Calif., in
the ratio of 1.0 gram of additive/coating per 2.0 grams of the
swollen superabsorbent material was applied to the superabsorbent
material. The coating additive was a mixture containing 0.5 grams
of glycerol and 0.5 grams of mineral oil for every 1.0 gram of
additive mixture plus 0.01 grams lecithin per 2.0 gram of swollen
superabsorbent material as an emulsifier. The lecithin was prepared
by grinding it to a fine powder for ten minutes and wetting
slightly with deionized water (about 2-3 milliliters) to aid in
mixing with the additive mixture. The lecithin was then added to
the additive mixture and mixed for about 30 minutes until a uniform
color with no observable lecithin particles was obtained. The
additive was then mixed into the superabsorbent material that was
previously swollen and had been equilibrating for one hour. The
additive mixture and the superabsorbent material were mixed for
about two minutes until there was little or no additive mixture
adhered to the side of the mixing bowl. The gel-bed friction angle
was measured as described above. The gel-bed friction angle of the
coated superabsorbent material at the given swelling level was
found to be 7 degrees and is summarized in Table 3.
Sample 6
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 gram of 0.9 weight percent of aqueous sodium
chloride solution per gram of superabsorbent material, and
equilibrated for one hour, as described above. A coating, mineral
oil (from Sample 2), glycerol (from Sample 1), and lecithin (from
Sample 5), in the ratio of 1.0 gram of additive/coating per 2.0
grams of the swollen superabsorbent material was applied to the
superabsorbent material. The coating additive was a mixture
containing 0.2 grams of glycerol and 0.8 grams of mineral oil for
every 1.0 gram of additive mixture plus 0.05 grams lecithin per 2.0
gram of swollen superabsorbent material as an emulsifier. The
lecithin was prepared by grinding it to a fine powder for ten
minutes and wetting slightly with deionized water (about 2-3
milliliters) to aid in mixing with the additive mixture. The
lecithin was then added to the additive mixture and mixed for about
30 minutes until a uniform color with no observable lecithin
particles was obtained. The additive was then mixed into the
superabsorbent material that was previously swollen and had been
equilibrating for one hour. The additive mixture and the
superabsorbent material were mixed for about two minutes and there
was little or no additive mixture adhered to the side of the mixing
bowl. The gel-bed friction angle was measured as described above.
The gel-bed friction angle of the coated superabsorbent material at
the given swelling level was found to be 2 degrees and is
summarized in Table 3.
Sample 7
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 grams of 0.9 weight percent of aqueous sodium
chloride solution per gram of superabsorbent material, and
equilibrated for one hour, as described above. A coating, mineral
oil (from Sample 2), glycerol (from Sample 1), and sorbitan
monolaurate, CAS 1338-39-2 (density 1.058 grams/cubic centimeter),
from Aldrich, in a ratio of 1.0 gram of additive/coating per 2.0
grams of the swollen superabsorbent material, was applied to the
superabsorbent material. The coating material/fluid was a mixture
containing 0.5 grams of glycerol and 0.5 grams of mineral oil for
every 1.0 gram of additive mixture plus 0.05 grams sorbitan
monolaurate per 2.0 gram of swollen superabsorbent material as an
emulsifier. The additive was then mixed into the superabsorbent
material that was previously swollen and had been equilibrating for
one hour. The additive mixture and the superabsorbent material were
mixed for about two minutes and there was little or no additive
mixture adhered to the side of the mixing bowl. The gel-bed
friction angle was measured as described above. The gel-bed
friction angle of the coated superabsorbent material at the given
swelling level was found to be 2 degrees and is summarized in Table
3.
Sample 8
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 grams of 0.9 weight percent of aqueous sodium
chloride solution per gram of superabsorbent material, and
equilibrated for one hour, as described above. A coating of
sorbitan monolaurate (from Sample 7) in the ratio of 1.0 gram of
additive per 2.0 grams of the swollen superabsorbent material was
applied to the superabsorbent material. The gel-bed friction angle
was measured as described above. The gel-bed friction angle of the
coated superabsorbent material at the given swelling level was
found to be 2 degrees and is summarized in Table 3.
TABLE-US-00004 TABLE 3 Gel-bed friction angle (in degrees) at
Example swelling level of 2.0 gram/gram Control 23 Sample 1 20
Sample 2 6 Sample 3 17 Sample 4 11 Sample 5 7 Sample 6 2 Sample 7 2
Sample 8 2
Sample 9
Three amounts of FAVOR.RTM. SXM 9543 were swollen to swelling
levels of 2 grams, 5 grams, and 10 grams, respectively, of 0.9
weight percent of aqueous sodium chloride solution per gram of
superabsorbent material and equilibrated for one hour, as described
above. A coating, mineral oil (from Sample 2), glycerol (from
Sample 1), and lecithin (from Sample 5), in the ratio of 1.0 gram
of additive/coating per 2.0 grams of the swollen superabsorbent
material was applied to each of the superabsorbent samples. The
coating additive was a mixture containing 0.5 grams of glycerol and
0.5 grams of mineral oil for every 1.0 gram of additive mixture
plus 0.01 grams lecithin per 2.0 gram of swollen superabsorbent
material as an emulsifier. The lecithin was prepared by grinding it
to a fine powder for ten minutes and wetting slightly with
deionized water (about 2-3 milliliters) to aid in mixing with the
additive mixture. The lecithin was then added to the additive
mixture and mixed for about 30 minutes until a uniform color with
no observable lecithin particles was obtained. The additive was
then mixed with one of the superabsorbent samples that had been
equilibrating for one hour. The additive mixture and the
superabsorbent material were mixed for about two minutes and there
was little or no additive mixture adhered to the side of the mixing
bowl. The gel-bed friction angle for each of the swelling levels
was measured as described above. The gel-bed friction angle of the
coated superabsorbent material at each of the given swelling levels
is listed in Table 4.
TABLE-US-00005 TABLE 4 Superabsorbent material swelling level
Gel-bed friction angle (in degrees) 2 grams/gram 7 5 grams/gram 6
10 grams/gram 4
Sample 10
Three amounts of FAVOR.RTM. SXM 9543 were swollen to swelling
levels of 2 grams, 5 grams, and 10 grams, respectively, of 0.9
weight percent of aqueous sodium chloride solution per gram of
superabsorbent material, and equilibrated for one hour, as
described above. A coating, mineral oil (from Sample 2), glycerol
(from Sample 1), and sorbitan monolaurate, (from Sample 8), in a
ratio of 1.0 gram of additive per 3.0 grams of the swollen
superabsorbent material, was applied to each of the superabsorbent
samples. The coating additive was a mixture containing 0.2 grams of
glycerol and 0.8 grams of mineral oil for every 1.0 gram of
additive mixture plus 0.02 grams sorbitan monolaurate per 2.0 gram
of swollen superabsorbent material as an emulsifier. The additive
was then mixed into each of the superabsorbent samples that had
been equilibrating for one hour. The additive mixture and the
superabsorbent material were mixed for about two minutes and there
was little or no additive mixture adhered to the side of the mixing
bowl. The gel-bed friction angle was measured as described above.
The gel-bed friction angle of the coated superabsorbent material at
each of the given swelling levels is listed in Table 5.
TABLE-US-00006 TABLE 5 Superabsorbent material swelling level
Gel-bed friction angle (in degrees) 2 grams/gram 5 5 grams/gram 4
10 grams/gram 4
Sample 11
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 grams of 0.9 weight percent of aqueous NaCl
solution per gram of dry superabsorbent material, and equilibrated
for one hour, as stated above. The first coating material of
mineral oil (from Sample 2), glycerol (from Sample 1), and lecithin
(from Sample 5) in a ratio of 1.0 gram additive/coating per 2.0
gram of swollen superabsorbent material was applied to the swollen
superabsorbent. The first coating material was a mixture of 0.495
grams mineral oil, 0.495 grams glycerol, and 0.01 grams Lecithin
per 1.0 gram additive/coating. The additive mixture and the
superabsorbent material were mixed and set aside to equilibrate for
30 minutes. Half of the material was used to measure the gel-bed
friction angle using the procedure as described above and the other
half was set aside for further treatment. The first treatment
gel-bed friction angle of the superabsorbent at the given swelling
level was found to be 15 degrees. The second half of treated
superabsorbent, that was previously set aside, was swollen to a
second swelling level of 10 grams of 0.9 weight percent of aqueous
NaCl solution per gram of dry superabsorbent material, and
equilibrated for one hour. A second coating was applied to the
treated swollen superabsorbent. The second coating material of
sodium silicate solution, available from Aldrich, a business having
offices in Milwaukee, Wis., in the ratio of 0.05 gram of additive
per 1.0 gram of swollen superabsorbent material was applied to the
swollen superabsorbent. The additive and the superabsorbent
material were mixed and set aside to equilibrate for 30 minutes.
The treated superabsorbent material was tested for gel-bed friction
angle as described above. The second treatment gel-bed friction
angle of the superabsorbent at the given swelling level of 10 gram
per gram was found to be 28 degrees, higher than what was measured
at 2 gram per gram.
Sample 12
Three amounts of FAVOR.RTM. SXM 9543 were swollen to swelling
levels of 2 grams, 5 grams, and 10 grams, respectively, of 0.9
weight percent of aqueous NaCl solution per gram of dry
superabsorbent material and equilibrated for one hour, as stated
above. A coating material of glycerol (from Sample 1) in a ratio of
1.0 gram additive/coating per 2.0 gram of swollen superabsorbent
material was applied to each of the swollen superabsorbent samples.
The gel-bed friction angle for each of the swelling levels was
measured as described above. The gel-bed friction angle of the
coated superabsorbent material at each of the given swelling levels
is listed in Table 6.
TABLE-US-00007 TABLE 6 Superabsorbent material swelling level
Gel-bed friction angle (in degrees) 2 grams/gram 20 5 grams/gram 15
10 grams/gram 14
Sample 13
Three amounts of FAVOR.RTM. SXM 9543 were swollen to swelling
levels of 2 grams, 5 grams, and 10 grams, respectively, of 0.9
weight percent of aqueous NaCl solution per gram of dry
superabsorbent material and equilibrated for one hour, as stated
above. A coating material of mineral oil, (from Sample 2), glycerol
(from Sample 1), and sorbitan monolaurate (from Sample 7) in a
ratio of 1.0 gram additive/coating per 2.0 gram of swollen
superabsorbent material was applied to each of the swollen
superabsorbent samples. The coating additive was a mixture
containing 0.8 grams of glycerol and 0.2 grams of mineral oil for
every 1.0 grams of additive mixture plus 0.01 grams of sorbitan
monolaurate per 1.0 grams of swollen superabsorbent material. The
additive was then mixed into the superabsorbent material
(previously swollen) for about 2 minutes and there was little or no
additive mixture adhered to the side of the mixing bowl. The
gel-bed friction angle for each of the swelling levels was measured
as described above. The gel-bed friction angle of the coated
superabsorbent material at each of the given swelling levels is
listed in Table 7.
TABLE-US-00008 TABLE 7 Superabsorbent material swelling level
Gel-bed friction angle (in degrees) 2 grams/gram 16 5 grams/gram 12
10 grams/gram 4
Sample 14
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 2 grams of 0.9 weight percent of aqueous NaCl
solution per gram of dry superabsorbent material, and equilibrated
for one hour, as stated above. A coating of glycerol (from Sample
2) in the ratio of 1.0 gram of additive per 2.0 grams of the
swollen superabsorbent material was applied to the swollen
superabsorbent material. The coated and swollen superabsorbent
material was dried in an oven at 90 degrees Celsius for 24 hours to
remove swelling fluid. The oven dried coated superabsorbent
material was re-swollen to a desired level of 2 grams of 0.9 weight
percent of aqueous NaCl solution per gram of coated superabsorbent.
The re-swollen superabsorbent material was tested for gel-bed
friction angle as described above. The gel-bed friction angle of
the superabsorbent at the given swelling level of 2 gram per gram
was found to be 12 degrees.
Sample 15
An amount of FAVOR.RTM. SXM 9543 was first swollen to a desired
swelling level of 10 grams of 0.9 weight percent of aqueous NaCl
solution per gram of dry superabsorbent material, and equilibrated
for one hour, as stated above. A coating of glycerol (from Sample
2) in the ratio of 1.0 gram of additive per 2.0 grams of the
swollen superabsorbent material was applied to the swollen
superabsorbent material. The coated and swollen superabsorbent
material was dried in an oven at 60 degrees Celsius for 5 days to
remove the swelling fluid. The oven dried coated superabsorbent
material was re-swollen to a desired level of 2 grams of 0.9 weight
percent of aqueous NaCl solution per gram of coated superabsorbent.
The re-swollen superabsorbent material was tested for gel-bed
friction angle as described above. The gel-bed friction angle of
the superabsorbent at the given swelling level of 2 gram per gram
was found to be 8 degrees.
While the embodiments of the present invention described herein are
presently preferred, various modifications and improvements may be
made without departing from the spirit and scope of the present
invention. The scope of the present invention is indicated by the
appended claims, and all changes that fall within the meaning and
range of equivalents are intended to be embraced therein.
* * * * *